Add String#delete_{prefix,suffix} to Lint/Void cop

[bdewater]

These methods were added in Ruby 2.5: https://ruby-doc.org/core-2.5.0/String.html#method-i-delete_prefix

---

Before submitting the PR make sure the following are checked:

• Wrote good commit messages.
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Run rake default or rake parallel. It executes all tests and RuboCop for itself, and generates the documentation.

[koic]

This should be written in Changes section instead of Bug fixes section.

[bbatsov]

Thanks!